Currently in hdd_extscan_start_fill_bucket_channel_spec() the
QCA_WLAN_VENDOR_ATTR_EXTSCAN_BUCKET_SPEC attribute is parsed without
specifying a policy. This means that no policy is enforced.
Subsequently the values of the nested attributes are retrieved, but
again without any length limits enforced. This could result in a
buffer overread.
To prevent this issue:
* Parse using the existing policy wlan_hdd_extscan_config_policy
* Update the policy to add missing attributes
Change-Id: I3b20cb28d1beccd2e804b022b531413ad1edb533
CRs-Fixed: 2054958
20b5dc36e3